2^00 CLASSIFICATION. 



and Huxley have demonstrated that the Pteropods are veritable 

 Gastropods furnished with a pair of accessory swimming organs. 

 Already the discoveiy of Grasteropteron has shown the little value 

 of the Pteropods as a division equivalent to the Gastropods. 

 The Heteropods merit Still less to be considered as a division 

 of equal value. 



The late Prof. 0. A. L. Morch, although attaching as much 

 systematic value to the lingual dentition as any other conchologist, 

 acknowledged that no single organ could be used in classification 

 unless its differential characters accorded with difl'erences of 

 other portions of the animal and shell ; but he endeavored to 

 show that conchologists have erred in estimating too highly for 

 systematic purposes the form of the shell, whilst neglecting 

 other external characters, such as sculpture, structure (nacreous, 

 porcellanous, etc.) and color. " According to my views, one 

 must consider shells, so to say, from a mineralogical point of 

 view." Having thus chosen conchological characters heretofore 

 neglected, in grouping the genera and families, the discrepancies 

 between a natural classification of the shells and one founded on 

 dentition, according to Prof. Morch, will disappear. " I have 

 united in the familj^ Tritonidse, according to the sculpture, 

 Ranella, Triton, Pyrula (Ficus). Dolium, Cassidaria and Cassis, 

 placing them near to Cyprsea." This is in accord with the 

 character of their dentition, which widely removes Triton and 

 Eanella from the Muricidse, close to which they have heretofore 

 been placed, upon conchological characters — principall}^ the form 

 of the shell, the presence of varices, the operculum, and also 

 a decided resemblance of the animals. 



I have care full}' re-examined these genera and their relation- 

 ships with others, in the point of view taken by Prof. Morch, 

 and the result of this examination is to convince me that he has 

 selected in the sculpture a character that is of generic importance 

 only in the single genus Dolium — that is to say, its species happen 

 to possess revolving ribs ; and even in sculpture the relationship 

 of Triton and Ranella with Murex is exceedingly close, whilst 

 they have little or no aflftnit}^ with Dolium, Cassis, etc. In fact, 

 it is precisely because Prof. Morch has regarded lingual denti- 

 tion a priori as an "infallible criterion " that he has been enabled 

 to detect supporting resemblances in the shells. It is easy to 

 show in man}'- other instances, as in the group under discussion, 

 how heterogeneous is the assemblage united by means of the 

 " infallible criterion." There is, besides, a growing conviction, 

 that there are no sharply-defined groups in nature ; that a generic 

 character, for example, cannot be made to cover all its species ; 

 that upon its borders occur forms which partake of the char- 

 acters of other so-called genera, and that families, orders, etc., . 



